In processes where metal surfaces come in contact with water, whether as liquid water or humid air, there is always the danger of corrosion. This is particularly problematic when the metal itself is prone to corrosion and is not coated.
Examples of metals prone to corrosion are found in stamped metal car parts made from ferrous alloys, abraded surfaces such as machined steel parts, and machine components made from cast iron.
Conversion coatings are used to inhibit corrosion and to assure good adhesion of paint to a metal surface, in particular to steel. Bare steel does not form a good bond with organic coatings. To improve the metal/organic coating bond, chromate or phosphate conversion coatings are chemically reacted with the steel to form a rough surface structure with metal-phosphate platelets that provide both mechanical and chemical keying (adhesion) of the organic polymer to the metal surface. See, for example, Watson, J. “A refresher: Understanding pretreatment”, Powder Coating 1996, 7(3) and Ferguson, D.; Monzyk, B., “Nonpolluting replacement for chromate conversion coating and zinc phosphate in powder coating applications”, Powder Coating 2001, 12(7). Here and hereinbelow, a conversion coating is to be understood to mean a protective barrier film or layer on the metal substrate surface simultaneously capable of both corrosion inhibition and paint adhesion and comprised of either (i) an organic film, (ii) an inorganic oxide resulting from a metal etching and redeposition process, (iii) an inorganic oxide resulting from a true deposition process, or (iv) any mixture of inorganic and organic materials from (i), (ii) and (iii). The most widely used conversion coatings are chromating, phosphochromating and zinc phosphatizing processes. It is believed that chromating gives better corrosion inhibition, while phosphochromating gives better paint adhesion. Prior to applying the conversion coating to the metal surface, the metal surface must be cleaned in order to remove any grease on the metal surface. The conversion coating can then be applied by either immersion or spraying. Chromate and phosphochromate conversion coatings are the most common chemical conversion coatings, but because of health, safety and environmental concerns other types of conversion coatings are needed.